The methods and systems utilized by the prior art to eliminate the vibrations generated in vehicular tires are basically modifications of balancing devices utilized in industry to balance industrial components, such as rotors, for the elimination of vibrations. The terms "balance" and "unbalance" are popularly used to indicate whether or not the part is free from vibration as it is rotated. In this context, the term "balance" denotes a condition wherein the rotor or the like is rotating about its principal axis and coincident with its supporting bearing.
In the case of a rotor, irrespective of its configuration, the unbalancing forces always occur once per revolution, which is the fundemental frequency of the rotating rotor. In the case of long flexible rotors, balancing is performed in multiple planes along the axis of the rotor. In all cases, however, the rotor, once balanced, has its principal axis of rotation rotating in coincidence with its bearings. Balancing can be readily accomplished since a rotor is effectively a homogenous, non-flexible mass, and it is always supported in bearings while freely rotating in space, whether during balancing or in actual use.
A rubber tire, on the other hand, although it is intended to be geometrically round, is not round when in use since it does not freely rotate in space. Rather, a tire must support a portion at least of the weight load of the vehicle, and while supporting the load radially, the tire is vertically in contact with the road. Unlike a rotor, which is effectively a solid mass, a tire is flexible and expands and contracts in use. The radius of a tire in the area of vertical contact with the road is always smaller than its radius at any other angle about the tire due to the load it carries. The area in contact with the road is known as the "tire patch" and it is the area which is vertically loaded (referred to as the "loaded radius"), the tire being deflected inwardly from 1/4 - 1/2 inch in the patch area under normal load and tire inflation. Consequently, as the tire revolves about the vehicle's wheel axle and bearings, which the tire also supports, the physical geometric center of the tire is never coincident with its principle bearing axis due to its constantly changing supporting radius. This condition creates a constantly changing dynamic disturbance which is directly related to the velocity (revolutions per minute) of the tire.
As a tire revolves, centrifugal forces are generated which increases dramatically with increases in velocity; and as the centrifugal forces increase, the tire tread gradually expands radially outwardly -- literally it grows -- and it also hardens due to the centrifugal outward forces acting on the tread rubber as well as on the fabrics or other reinforcement incorporated in the tire tread. As the tire tread diameter grows, its over-all structure stiffens and the size of the tire patch is reduced, along with a reduction in sidewall bulginess, resulting in an increase in the loaded radius of the tire which acts to raise the vehicle. In addition, there are many variables which enter into tire construction, including variations caused by the various splices in the tire; variations in the materials from which the tire is constructed, including variations in their composition; dimensional irregularities and inaccuracies both in the tire building equipment and in curing; and, of course, human error. The net result is the generation of a complexity of ramdomly distributed vibration producing forces which are transmitted to the vehicle through its wheel hubs and axles. These vibrations producing forces manifest themselves as non-circular areas (non-uniformities) extending around the periphery of the tire. These non-uniformities vary in both location and magnitude depending upon velocity, the velocity of particular concern being the cruising speed at which the vehicle normally will be operated. At crusing speeds the resiliency of the tire is greatly reduced, which amplifies the stiffness of the non-circular area as they roll along the road. Since the size of the tire patch is also reduced, the unit load is greater, which magnifies the reaction between the non-uniformities and the road surface. This reaction results in a rapidly changing loaded radius which generates a complex up and down movement of the wheel hub and axle for each revolution of the tire as the non-uniformities come into contact with the road surface, and these movements are transmitted as vibrations to the vehicle.
Although the terms "balance" and "unbalance" also have been extensively used in the tire manufacturing and correction art, the terms are misleading and generally apply only to a small spectrum of the vibration producing forces, usually being confined to the forces produced by lack of tire symmetry which occur only once per revolution of the tire and are variable in magnitude, depending upon velocity. It may be flatly stated that all tires leaving the tire manufacturer's plant are in an over-all self-exciting condition, that is, they are not free from vibration when in use. Even though various "balancing" steps may be performed by the automobile manufacturer or by the seller or user of the tires which may improve their balance, the corrected tires are not free from vibration causing forces which are centrifugally generated either at the fundamental frequency or at frequencies of higher order. While certain prior art processes have included the honing of the outside periphery (tread) of the tire to improve its roundness and concentricity, the areas of the tread to be honed are determined under essentially static conditions, usually without loading to simulate the weight load placed on the tire in use. Even in prior art systems wherein the tire is loaded and revolved at speeds of approximately one to five miles per hour, the physical eccentricity, if any, generated in the tire at these low speeds is quite different from the centrifugally generated eccentricity encountered when the tire is revolved under true environmental-like conditions, i.e., under load and at cruising speed. Suffice to say that the prior art, while recognizing the existence of lateral and radial force variations when a tire is revolved in space and the desirability of compensating for these variations, has totally failed to understand the true nature of the centrifugally developed forces and their effect on tire performance, much less how to effectively correct the tire to reduce these forces to an acceptable low level. The centrifugal forces developed in a tire at relatively slow speeds, such as 50 or even 100 revolutions per minute, are normally neglible and have no real effect on either tire non-uniformity or dynamic unbalance. The vibration producing forces which cause the real problem are those which occur at relatively high speeds, normally crusing speed, and it is these forces which have been wholly neglected by the prior art.
Prior art systems are known wherein "balancing" is effected by adding weights to the wheel rim, or by adding counterbalancing weights to the tire itself. Some add the weights to the inside and others to the outside of the tire. In one system the weight is added internally to the entire crown of the tire and then removed as required. In all instances, however, the correction attempted is not dictated by realistic changes in tire configuration caused by high angular centrifugal forces; rather, it is dictated by forces encountered when the tire is in a static or near-static condition. It may be additionally noted that where the weight is added internally to the crown of a tire, the performance characteristics of the tire are altered. As in the case where material is honed from the periphery of the tire when in a static or near-static condition, the location and amplitude of the generated forces will change with changes in velocity; consequently the addition or substraction of material from the crown of the tire may worsen rather than improve vibration when the tire is operated at cruising speed.
In order to correct the tire both for peripheral non-uniformities and for lack of symmetry, it is essential to locate and measure, as well as segregate, the vibration producing forces. To this end, the present invention utilizes the measuring system disclosed in co-pending U.S. Pat. No. 3,862,570, dated Jan. 28, 1975, in the name of the present inventor. In accordance with the teachings of this patent, both the dynamic unsymmetrical forces and the centrifugally generated forces which are the result of radially loaded peripheral non-uniformities are simultaneously measured and recorded so that the appropriate corrective measures may be taken.